The invention relates generally to ion beam devices and instruments. In particular, the invention relates to methods and apparatus for storing and ejecting ions.
Quadrupole ion traps are ion storage devices that are commonly used in mass spectrometers. Quadrupole ion traps have electrodes that are shaped to produce a three-dimensional quadrupolar field that stores ions. Many known quadrupole ion traps have hyperbolic shaped electrodes, which gives rise to a three-dimensional quadrupolar field. Generally, quadrupole ion traps are designed and operated to store only ions having a charge-to-mass ratio within a certain range.
Quadrupole ion traps can be configured and operated to simultaneously eject all of the stored ions. Quadrupole ion traps can also be configured and operated to periodically eject ions according to their charge-to-mass ratio as a function of time to produce a mass spectrum of ions. For example, periodic ejection of ions according to their charge-to-mass ratio can be accomplished by varying the voltages applied to the ion trap as a function of time to eject ions having differing charge-to-mass ratios. The ions are ejected out of the storage field and into an ion detector. The ion detector counts the ejected ions or measures the ion flux of the resulting ion beam. Known ion traps generally produce mass spectrums of ions that are heavier than Argon.
An ion storage system is described that can be used in a mass spectrometer or in a leak detector. In one embodiment, an ion storage system according to the present invention is used in a mass spectrometer-based leak detector. An ion storage system according to the present invention ejects ions by abruptly terminating the RF field trapping the ions. Abruptly terminating the RF field causes substantially all of the ions to be ejected in a relatively short period of time, which generates a relatively high ion current signal. In one embodiment, the ion current of ions ejected from the ion storage system is measured at a time that is synchronized to electrical events associated with the ion storage systems, such as the termination of the RF field, injection of the sample gas, and interrupting the operation of electrical noise producing devices such as vacuum pumps.
Accordingly, the present invention features an ion storage system that includes an ion trap that defines a volume for storing a plurality of ions. In one embodiment, the ion trap forms a substantially cylindrically shaped volume. In another embodiment, the ion trap forms a volume having substantially curved walls. The curved walls can be substantially hyperbolic in shape.
The ion storage system further includes a radio frequency (RF) generator that is electromagnetically coupled to the volume defined by the ion trap. The RF generator generates a RF electrical field that stores the plurality of ions in the ion trap. The ion storage system also includes a switching device that terminates the RF electrical field. The termination of the RF electrical field causes the plurality of ions to be ejected from the ion trap.
In one embodiment, the switching device is an electronic switching device. In another embodiment, the switching device is a mechanical or electro-mechanical switching device, such as a relay. In one embodiment, the switching device causes a short circuit condition that terminates the RF electrical field. The switching device can terminate the RF electrical field within a time period that is substantially less than or equal to one cycle of the RF electrical field. In one embodiment, the switching device is substantially synchronized with a predetermined phase of the RF electrical field. In one embodiment, a clock synchronizes the switching device. In one embodiment, the clock synchronizes the switching device to the ion detector. Alternatively, the clock can determine a time at which the switching device terminates the RF electrical field.
In one embodiment, the ion storage system includes an ion source that generates the plurality of ions. In one embodiment, the ion source provides the plurality of ions to the ion trap. In another embodiment, the ion source generates the plurality of ions in the volume defined by the ion trap. In one embodiment, the ion source includes an electron source. For example, the electron source can include a thermionic emission filament. In one embodiment, the ion source includes a gas injector, which may be a pulsed gas injector, which provides neutral gas molecules or atoms to the ion source.
The ion storage system also includes an ion detector. In one embodiment, the ion detector includes an electron multiplier. In one embodiment, the ion detector is substantially synchronized to the switching device. The ion detector is adapted to detect at least a portion of the plurality of ions that are ejected from the ion trap. In one aspect of the invention, the ion detector is substantially synchronized to the generation of the ions by the ion source. In another aspect, the ion detector is substantially synchronized with a predetermined phase of the RF electrical field. In yet another aspect of the invention, the ion detector is substantially synchronized to the interruption of sources of electrical noise, such as the filament power supply and the vacuum pump motor power supply.
The present invention also features an ion storage system that includes an ion source that generates a plurality of ions, an ion trap that defines a volume for storing the plurality of ions, a radio frequency (RF) generator that is electromagnetically coupled to the volume defined by the ion trap, an ion detector that detects at least a portion of the plurality of ions that are ejected from the ion trap, and a clock that synchronizes events occurring within the ion storage system. In one embodiment, the ion source generates the plurality of ions in the ion trap. In one embodiment, the ion source includes an electron source and a pulsed gas source.
In one embodiment, the ion trap forms a substantially cylindrically shaped volume. In another embodiment, the ion trap forms a volume having substantially curved walls. The curved walls can be substantially hyperbolic in shape. The RF generator is adapted to generate a RF electrical field that stores the plurality of ions in the ion trap. The ion storage system also includes a switching device that terminates the RF electrical field, thereby ejecting the plurality of ions from the ion trap. The ion detector detects at least a portion of the plurality of ions that are ejected from the ion trap.
The clock is electrically connected to at least two of the ion source, the RF generator, the switching device, and the ion detector. The clock is adapted to substantially synchronize at least two of the ion source, the RF generator, the switching device, and the ion detector. In one embodiment, the detector includes an electron multiplier. In another embodiment, the ion detector is substantially synchronized to the interruption of sources of electrical noise, such as the filament power supply and the vacuum pump motor power supply.
The present invention also features a method for detecting ions. In one embodiment, the method includes generating ions from neutral gas molecules or atoms. The method also includes establishing a radio frequency (RF) electrical field proximate to a plurality of ions, thereby trapping the plurality of ions in a volume.
In one embodiment, the RF electrical field is terminated, thereby ejecting the plurality of ions from the volume. At least a portion of the plurality of ions ejected from the volume is detected at a predetermined time after terminating the RF electrical field. In one embodiment, a time at which the RF electrical field is terminated is substantially synchronized to at least one of a predetermined phase of the RF electrical field and a time of detecting at least a portion of the ions ejected from the volume.
In one embodiment, the termination of the RF electrical field is completed substantially within one cycle of the RF electrical field. In another embodiment, the detection of the portion of the plurality of ions ejected from the volume occurs at a predetermined time after the termination of the RF electrical field. In another embodiment, the detection of the portion of the ions ejected from the ion trap occurs at a predetermined time after the termination of the RF electrical field that maximizes a signal-to-noise ratio of an electrical signal related to the detection of the portion of the ions. In yet another embodiment, the termination of the RF electrical field comprises establishing a short-circuit condition that terminates the RF electrical field.
The present invention also features a leak detector. The leak detector includes an ion source that receives a tracer gas and that generates a plurality of ions of the tracer gas. An ion trap defines a volume for storing the plurality of ions of tracer gas. A radio frequency (RF) generator is electromagnetically coupled to the volume defined by the ion trap. The RF generator generates a RF electrical field that stores the plurality of ions of the tracer gas in the ion trap.
The leak detector includes a switching device that terminates the RF electrical field. The termination of the RF electrical field ejects the plurality of ions from the ion trap. An ion detector detects at least a portion of the plurality of ions that are ejected from the ion trap. A clock is electrically connected to at least two of the ion source, RF generator, the switching device, and the ion detector. The clock substantially synchronizes at least two of the ion source, the RF generator, the switching device, and the ion detector.